At present, as a resin for powder coating, an epoxy resin, a polyester resin, a polyacrylic resin, a polyvinyl chloride resin or a polyethylene resin is, for example, used.
In recent years, a thermosetting resin is mainly used from the viewpoint of the smoothness and physical properties of the coating film. However, a thermosetting resin has a drawback that when a condensation type resin is used or when a blocked isocyanate is used as a curing agent, a condensation-liberated component or a blocking agent will form a gas at the time of curing, which in turn forms bubbles in the coating film.
Therefore, as a curing agent preferred for powder coating, a polyglycidyl compound may, for example, be mentioned which is free from a component liberated during the curing reaction. However, when a usual glycidyl compound such as a bisphenol type diglycidyl ether, is used as a curing agent, a bisphenol type diglycidyl ether of a low molecular weight type is liquid at room temperature, and if it is melt-mixed with a carboxyl group-containing resin, followed by pulverization to obtain a powder coating, it brings about a problem that the powder undergoes fusion or blocking during the storage.
On the other hand, a bisphenol type diglycidyl ether of a high molecular weight type has a less number of functional groups (epoxy groups) per unit weight, and a larger amount will be required, which is disadvantageous from the viewpoint of the cost. Further, if a bisphenol type diglycidyl ether of a high molecular weight type is incorporated in a large amount, the melt viscosity of the coating material tends to be high, thus leading to a problem that smoothness of the coating film will be impaired.
Namely, the curing agent is usually desired to be solid at room temperature and have a low melt viscosity and a large number of functional groups per unit weight. As such a curing agent, a compound having three glycidyl groups or similar functional groups per molecule, such as triglycidyl isocyanurate or tris(.beta.-methylglycidyl) isocyanurate, may, for example, be mentioned.
Triglycidyl isocyanurate has high crystallizability and thus has a high melting point at a level of from 100 to 140.degree. C. Accordingly, in order to mix it uniformly with the resin containing carboxyl groups, it will be necessary to conduct the kneading at a slightly high temperature (a temperature around the melting point). In addition, this triglycidyl isocyanurate is highly reactive, and a curing reaction between carboxyl groups and glycidyl groups tends to proceed to some extent to form powder coating even at the temperature for the kneading, whereby the flow property (fluidity) at the time of baking (heat curing) tends to decrease slightly, thus leading to formation of surface roughening so-called orange peel on the surface of the coating film.
On the other hand, tris(.beta.-methylglycidyl) isocyanurate has a melting point of from 70 to 100.degree. C., and it can be uniformly kneaded with the resin containing carboxyl groups at a relatively low temperature at a level of 100.degree. C. or lower, whereby the curing reaction during the kneading can be suppressed. Therefore, it is free from the above-mentioned problem.
As a case wherein this tris(.beta.-methylglycidyl) isocyanurate is employed, Japanese Unexamined Patent Publication No. 24244/1974 discloses a resin composition for powder coating, which comprises tris(.beta.-methylglycidyl) isocyanurate and a polyester resin having an acid value of from 30 to 200 prepared from an aromatic dicarboxylic acid or its ester, a dihydroxy alcohol and a polybasic carboxylic acid.
Japanese Unexamined Patent Publication No. 94722/1974 discloses a method for producing a resin composition for powder coating comprising a free hydroxyl group-containing polyester resin, an acid anhydride and an epoxy compound such as tris(.beta.-methylglycidyl) isocyanurate.
Japanese Unexamined Patent Publication No. 19832/1975 discloses a method for producing a resin composition for powder coating comprising a polyester resin containing a carboxyl group via an ester bond on a side chain and an epoxy compound such as tris(.beta.-methylglycidyl) isocyanurate.
Japanese Unexamined Patent Publication No. 44130/1976 discloses a resin composition for powder coating, which comprises a polyester resin having free phenolic hydroxyl groups, a compound having at least two glycidyl groups and having a melting point of from 30 to 250.degree. C., and/or an epoxy compound such as tris(.beta.-methylglycidyl) isocyanurate, and/or a monomer radical-copolymerizable with a compound having an epoxy group. This composition may contain, as a catalyst, a tetraalkylammonium salt such as tetrabutylammonium bromide or tetraethylammonium chloride, an imidazole compound such as imidazole, 2-methyl-4-ethyl-imidazole or 2-methyl-imidazole, a tertiary amine such as triethylene diamine or N-methylmorpholine, or an organic carboxylic acid compound such as cobalt naphthenate or zinc naphthenate.
Japanese Unexamined Patent Publication No. 12823/1976 discloses a composition for powder coating having a polybasic carboxylic acid incorporated to a polymer made of a vinyl type monomer having a (methyl)glycidyl group, an unsaturated polyester having (methyl)glycidyl groups and other vinyl type monomers. To this composition, an acid, an alkali or an amine may be added in order to promote the curing reaction.
Japanese Unexamined Patent Publication No. 69935/1977 discloses a composition for powder coating having a certain polyester having a low acid value incorporated to a carboxyl group-containing polyester and triglycidyl isocyanurate or tris(.beta.-methylglycidyl) isocyanurate.
Japanese Unexamined Patent Publication No. 63872/1992 discloses a composition for powder coating, which comprises a carboxyl group-containing resin and a polyglycidyl compound having from 2 to 6 glycidyl groups per molecule obtained by adding an epoxy compound such as tris(.beta.-methylglycidyl) isocyanurate to a polyester oligomer having at least two carboxyl groups per molecule. This composition may contain a curing catalyst such as imidazole.
Japanese Unexamined Patent Publication No. 288373/1992 discloses a composition for powder coating, which comprises a basic resin and a curing agent, and further a curing catalyst. The basic resin is a resin having at least two carboxyl groups per molecule. As the curing agent, triglycidyl isocyanurate or a polyglycidyl compound having from 2 to 6 glycidyl groups, on average, per molecule obtained by adding tris(.beta.-methylglycidyl) isocyanurate to a polyester oligomer having at least two carboxyl groups per molecule, is disclosed. Further, as the curing catalyst, a tertiary amine such as triethylamine, or an imidazole compound having a secondary amino group, such as imidazole or 2-ethylimidazole, is disclosed.
When an epoxy compound such as triglycidyl isocyanurate or tris(.beta.-methylglycidyl) isocyanurate is added as a curing agent to a polyester resin having free hydroxyl groups, particularly free phenolic hydroxyl groups (--C.sub.6 H.sub.4 OH) as disclosed in Japanese Unexamined Patent Publications No. 94722/1974 and No. 44130/1976, to obtain a composition for powder coating, ether bonds formed by the reaction between phenolic hydroxyl groups and the above epoxy resin are not desirable as the weather resistance is poor, since the stability of phenoxy groups (--C.sub.6 H.sub.4 O--) is high, and the phenoxy groups tend to readily be liberated by light. Further, the polyester resin having free phenolic hydroxyl groups, has a low reactivity with the epoxy compound added as a curing agent, since the influence of H.sup.+ (protons) attributable to the phenolic hydroxyl groups is small. Accordingly, for the purpose of increasing the reactivity, it is common to employ a method of adding a cure-promoting agent such as a tertiary amine.
On the other hand, Japanese Unexamined Patent Publications No. 24244/1974, No. 19832/1975, No. 69935/1977, No. 63872/1992 and No. 288373/1992 disclose compositions for powder coating having an epoxy compound such as triglycidyl isocyanurate or tris(.beta.-methylglycidyl) isocyanurate added as a curing agent to a carboxyl group-containing resin.
In a composition for powder coating comprising a carboxyl group-containing resin and a curing agent having a glycidyl group, if triglycidyl isocyanurate is used as a curing agent for such a composition for powder coating, at the time of heat curing on a substrate, glycidyl groups in the triglycidyl isocyanurate have a high reactivity with carboxyl groups, and thus the carboxyl groups and the glycidyl groups react efficiently, whereby the desired curing reaction proceeds. However, in a case where tris(.beta.-methylglycidyl) isocyanurate is employed as a curing agent for the resin containing carboxyl groups, homo-polymerization due to ring opening polymerization among .beta.-methylglycidyl groups themselves is likely to take place at the time of heat curing on a substrate, whereby the desired curing reaction between carboxyl groups and .beta.-methylglycidyl groups tends to hardly take place. Consequently, the curing will be inadequate, and the resulting coating film will have inadequate physical properties with respect to impact resistance, weather resistance, etc. It is considered that with the carboxyl group-containing polyester resin, the influence of H.sup.+ (protons) attributable to the carboxyl groups is so large that homo-polymerization due to ring opening polymerization among .beta.-methylglycidyl groups themselves takes place preferentially.